1. Field of the Invention
The present invention relates to the transmission of electrical signals and, more particularly, it relates to the design and fabrication of transmission lines on multiple layer circuit structures such as printed circuit boards.
2. Discussion of the Prior Art
Printed circuit boards (PCBs) are employed as a foundation for the mounting of various electronic components making up a circuit or system. With the short signal transition times and high clock rates of modem digital circuitry, the electrical properties of the electrical conductors carrying signals between devices become increasingly more important. High-speed logic families such as FAST, ECL and GaAs circuits have required the PCB designer to model the aforementioned electrical signals as transmission lines. Indeed, PCB layout is now deemed very critical to maintaining signal integrity, such as preserving signal edges and reducing distortion due to reflections and crosstalk. Proper impedance control and impedance termination are required in order to achieve desired signal integrity.
Essentially, the impedance of a printed circuit board (PCB) conductor is controlled by its configuration, dimensions (trace width and thickness and height of the board material) and the dielectric constant of the board material. When a signal encounters a change of impedance arising from a change in material or geometry, part of the signal will be reflected and part transmitted. Such reflections are likely to cause aberrations in the signal which may degrade circuit performance (e.g., low gain, noise, and random errors). Each transmission line on a PCB is generally formed by two conductive traces. If one of the two conductors of the transmission line coupling the circuits is used as a grounding connector, the circuit is referenced as single-ended and the transmission line is a single-ended or unbalanced transmission line. Otherwise, the two non-grounded conductors form a differential or balanced transmission line for a differential circuit. Equivalent examples of differential transmission lines are shown in FIG. 1A in which the two conductors 2a and 2b are implemented as wire rods separated by a dielectric material 4 within a grounded sheath 6xe2x80x94and in FIG. 1Bxe2x80x94in which the two conductors are 2a and 2b are implemented as conductive traces formed by metallizations on dielectric layers 8a and 8b between ground planes 10a and 10b. 
Although all electrical circuits depend on a difference in voltage for their operation, differential signaling refers to where two signals are sent and received with the information conveyed by their different in voltage or current rather than by those individual quantities with respect to a common ground. A differential signal is applied across the two conductors, as voltage varying signals xe2x88x92V and +V applied to conductors 2a and 2b in FIGS. 1A and 1B, by a differential generating circuit (not shown), the signal travels down the transmission line to a differential receiving circuit (not shown), and the received signal is measured as the difference between the voltage or current in conductors 2a and 2b. In other words, a differential circuit generates or receives a pair of complementary signals in a phase inverted relation with each other, known together as a single differential signal.
The chief advantage of differential signaling is higher noise immunity, achieved by eliminating common-mode influences picked up by the environment. Provided its conductive traces are close enough together to be exposed to nearly the same environmental influences, the differential transmission line is desensitized to voltage drift because the induced noise in each line rise and fall in tandem in the complementary signals and cancel each other. The inherently superior noise immunity of differential circuits allows signal levels to be reduced, with consequent improvements in switching speed, power dissipation and noise. Another advantage lies in the area of ground bouncexe2x80x94since differential pairs always sink and source the same amount of current (into a resistive load), there is no bounce and, in theory, no bounce-induced crosstalk. And since the currents are complementary there is little net magnetic flux from a differential pair, so EMI is dramatically reduced as well.
Despite the aforementioned benefits of configuring PCB traces interconnecting circuit devices as differential transmission lines, complications do arise in the application of this technique to electrical systems in which many signals need to be conveyed wherein it is desired to employ multiple sets of differential lines. An illustrative electrical board 12 embodying the approach is shown in FIG. 2, with three differential transmission lines indicated generally at 14, 16 and 18, being constructed using multiple planes of metallizations over corresponding dielectric layers 20 and 22. Upper ground plane 24 is spaced the same distance above upper conductors 14a, 16a and 18a of transmission lines 14, 16 and 18 as lower ground plane 26 is separated from lower conductors 14b, 16b and 18b, so as to have equal impedances to ground. As between a first differential transmission line as differential conductor pair 14a and 14b and a second differential transmission line as differential conductor pair 16a and 16b, the problem of crosstalk can arise in which a signal on the first line can induce a voltage on the second line and vice versa.
It is well known that the most effective way to improve local PCB crosstalk between adjacent differential transmission lines is to move the affected traces further away. Assuming one is presented with solid power and ground planes, crosstalk between aggressor and victim traces falls off as the square of increasing distances. Thus, for example, doubling the distance cuts crosstalk to one-fourth. While placing the two traces of a line pair closer together also helps reduce crosstalk, the greatest effect is achieved by generally increasing the separation between aggressor and victim. Unfortunately, however, space is generally limited on printed circuit board structures and design considerations tend to require that the differental transmission lines be placed as closely together as practicable. Accordingly, a need arises for a technique for realizing differential transmission lines as conductive traces on a PCB in which space on the board is used more efficiently than has heretofore been possible without unacceptable degradation in performance due to crosstalk. A further need arises for a PCB which may be fabricated using conventional manufacturing techniques and equipment.
The aforementioned needs are addressed, and an advance is made in the art, by a multilayer circuit structure which includes first and second superposed planar dielectric layers. On the first layer, first and second conductor lines are formed, and on the second layer, third and fourth conductor lines are formed. The first and third conductor lines, and the second and fourth conductor lines, respectively, form at least a portion of corresponding first and second differential transmission line. The first conductor line extends in parallel with the third conductor line but is both vertically spaced and horizontally offset therefrom such that a first plane orthogonal to and extending through the first and second planar layers extends through one, but not both of the first and third conductor lines (i.e., a non-overlapping region). Similarly, a second plane orthogonal to and extending through the first and second planar layers extends through a non-overlapping region of the second and fourth conductor lines.
The dimensions and electrical properties of the first and third conductor lines are selected so that the characteristic impedance of the first transmission line is matched to the output of a differential transmission circuit which is operable to generate and supply to the first and third conductor lines a pair of complementary current or voltage varying signals in phase inverted relation to one another. Additionally, the characteristic impedance of the first transmission line is matched to the input of a differential receiving circuit configured to measure a difference between the pair of complementary voltage or current varying signals received in phase inverted relation to one another from the differential transmission circuit. Illustratively, the characteristic impedance of the first transmission line may be from about 80 to about 110 ohms.
A method of reducing crosstalk between adjacent differential transmission lines in a multilayer circuit structure comprises the steps of applying a first signal to a first differential transmission line, and applying a second signal to a second differential transmission line adjacent the first differential transmission line, wherein a first electrically conductive element of the first differential transmission line is angularly offset from a second electrically conductive element of the first differential transmission line by a rotation angle selected to result in less crosstalk being induced in the second differential transmission line than would be induced if the first and second electrically conductive elements were aligned in either an over-under orientation in a vertical plane or in a side-by-side orientation in a horizontal plane.